Ablation in Liquids: Structure and Sizing Synthesis of Ni ...

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Article

Synthesis of Ni Nanoparticles by Fs LaserAblation in Liquids: Structure and Sizing

David Muñetón Arboleda, Jesica María Jose Santillán, Luis Joaquín Mendoza Herrera, Marcela B.Fernández van Raap, Pedro Mendoza Zélis, Diego Muraca, Daniel Schinca, and Lucía Scaffardi

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03124 • Publication Date (Web): 13 May 2015Downloaded from http://pubs.acs.org on May 15, 2015

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Synthesis of Ni Nanoparticles by Fs Laser Ablation in Liquids: Structure and Sizing David Muñetón Arboleda,† Jesica M. J. Santillán,†,‡ Luis J. Mendoza Herrera,† Marcela B. Fernández van Raap,ǂ,ǁ Pedro Mendoza Zélis,ǂ,ǁ,§ Diego Muraca,# Daniel C. Schinca,†,§ and Lucía B. Scaffardi*,†,§

†Centro de Investigaciones Ópticas (CIOp), CONICET La Plata < CIC, Camino Centenario y 506, 1897 Gonnet, La Plata, Argentina ‡Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Catamarca, Av. Belgrano 300, 4700 San Fernando del Valle de Catamarca, Catamarca, Argentina ǂInstituto de Física La Plata (IFLP), CONICET < UNLP, Calle 49 y 115, 1900 La Plata, Buenos Aires, Argentina ǁDepartamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Calle 49 y 115, 1900 La Plata, Buenos Aires, Argentina §Departamento de Ciencias Básicas, Facultad de Ingeniería, Universidad Nacional de La Plata, Calle 115 y 49, 1900 La Plata, Buenos Aires, Argentina #Instituto de Física “Gleb Wataghin” (IFGW), Universidade Estadual de Campinas, Rua Sérgio Buarque de Holanda 777, Cidade Universitária Zeferino Vaz Barão Geraldo CEP 13083<859, Campinas, São Paulo, Brasil Supporting Information

ABSTRACT

Synthesis of nickel (Ni) nanoparticles (NPs) suspensions was performed using a

120 femtosecond (fs) pulse laser to ablate a Ni solid target in n<heptane and water.

Analysis of structure, configuration and sizing was carried out using different

independent techniques such as Optical Extinction Spectroscopy (OES), Atomic Force

Microscopy (AFM), Transmission Electron Microscopy (TEM) and Electron

Diffraction (ED), which yield interrelated information.

AFM microscopy allows determining the spherical shape and size distribution of

the NPs in the obtained colloids while TEM provides knowledge about shape, structure

and size distribution. ED allows identification of the different metal and metal oxide

composition as well as their crystallographic phase. On the other hand, OES gives

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information related to size distribution, structure, configuration and composition.

Interpretation of these spectra is based on Mie theory which, in turn, depends on Ni

dielectric function. For NPs radii smaller than 3 nm, size<dependent free and bound

electron contributions to the dielectric function must be considered. To account for the

full size span, complete Mie expansion was used for optical extinction cross<section

calculations. A theoretical analysis of the dependence of plasmon resonance of bare

core and core<shell Ni NPs with core size and shell thickness provide insight about their

spectroscopic features.

For n<heptane, species like bare core Ni and hollow Ni NPs are found in the

colloid, being the latter reported for the first time in this work. Instead, for water, the

colloid contains hollow nickel NPs and nickel oxide in different core<shell

configurations: Ni<NiO and NiO<Ni, being the latter also reported for the first time in

this paper. In both cases, size distribution agrees with that derived from TEM and AFM

analysis. The formation of the oxide species is discussed in terms of oxidation<reduction

processes during ablation. Possible mechanisms for the formation of hollow species are

proposed.

KEYWORDS: Ni Nanoparticles, Femtosecond Laser Ablation, Optical Extinction

Spectroscopy, Size<Dependent Dielectric Function, Optical Properties, Plasmon

Resonance

I. INTRODUCTION

Metal NPs studies became the focus of intense research owing to their unusual

properties compared with bulk metal, which convey many potential applications in

different science and technology fields.1 The control of physical, chemical and

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biological properties in the nanometric scale is based on the ability to manipulate the

size and shape of NPs.2–7

In recent years, Ni NPs have attracted scientific interest because of their

extensive prospects in catalysts,8, 9 conducting pastes,10 information storage,8 large<scale

batteries,11 magnetic behavior,12–15 enhanced optical properties,16 and biomedicine.17

Several works on Ni NPs generation by laser ablation using different pulsed

laser regimes and different media have appeared in the literature in the last years. Liu et

al. studied the morphological and structural properties of Ni NPs generated by 500 fs

pulse laser ablation in low pressure background gas.18 They determined that by limiting

the laser fluence below the threshold of plasma formation, large particles may be

avoided. Besides, by varying the background gases, Ni<NiO core<shell spheres were

obtained. Amoruso et al. used 300 fs laser pulses focused on a Ni target in vacuum to

study the expansion dynamics of the ablated species by fast photography and optical

emission spectroscopy.19 Dudoitis et al. generated Ni NPs in argon and air using a

Nd:YAG laser working in two different pulsed regimes, 15 ns and 10 ps, and describing

the influence of laser intensity and thermal capacities on the electron and lattice

temperature.20

On the other hand, generation of Ni NPs via laser ablation in liquids has been

recently reported by Jaleh et al., who worked with a high repetition Nd:YAG laser of

240 ns pulse width for ablating a pure Ni plate immersed in methanol with PVP as

stabilizer agent, studying their localized surface plasmon via optical spectroscopy.21

Jung and Choi have worked with a Q<switched Nd:YAG laser at its fundamental

wavelength to ablate Ni target in different solvents and have studied their final

crystallographic phase, concluding that it is related to the thermodynamic factors of the

laser<induced plasma plume and to the specific heat of the solvents.22 Musaev et al.

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fabricated colloids of Ni in water by laser ablation with a nitrogen laser of 10 ns pulse

duration and determined its composition using X<ray diffraction (XD).23

However, there are no works reporting on the fabrication and characterization of

colloids obtained using fs<regime laser ablation of Ni in liquids. In this work we analyze

the characteristics of structure (bare core or core<shell), configuration, number density

and size distribution of NPs generated when a fs pulse laser is used to ablate a Ni solid

target in n<heptane and water. Optical extinction was used to obtain the spectroscopic

characteristics of freshly fabricated suspensions. This technique is appropriate for this

type of samples since it probes a statistically relevant number of particles (about 1010 to

1011 particles) and can yield qualitative and quantitative information about particle size

distribution and number density. Extinction spectra were modeled using Mie theory24

through an appropriate modification of the bulk complex dielectric function, taking into

account a size<dependent corrective term for each free and bound electron contribution.

We theoretically explore the dependence of extinction spectrum with size for bare core

and core<shell Ni NPs. This spectrum is dominated by plasmon resonance whose

wavelength is dependent on size, shell thickness and surrounding media.

Here we report for the first time the synthesis of Ni NPs suspensions using a

120 fs pulse laser to ablate a Ni solid target in n<heptane and water, the presence of

NiO<Ni core<shell and hollow Ni NPs in these colloids and propose a possible

mechanism for the formation of different species present in the obtained suspensions.

AFM, TEM and ED analyses independently performed on the colloid samples yielded

size and structural results that agree with those obtained by OES.

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II. EXPERIMENTAL SECTION

Nickel colloidal suspension was fabricated by fs pulse laser ablation. The target

sample used to carry out these experiments was a 1 mm thick solid disk of high purity

grade nickel, immersed in a vessel filled with 4 cm3 of n<heptane or milli<Q water. This

solvent volume created a liquid column of 1 cm height over the Ni disk. Laser ablation

was performed using a Ti:Sapphire (Ti:Sa) chirped pulsed amplification (CPA) system

from Spectra Physics, emitting pulses of 120 fs with at 1 kHz repetition rate centred at

800 nm wavelength. The maximum output energy was 1 mJ per pulse, but it could be

attenuated using a classical waveplate<polarizer system. A 5 cm focal length lens was

used to focus the laser beam on the target disk surface. The pulse energy used in this

experiment was 100 �J. Considering the focal length of the focusing lens and the input

beam diameter, the corresponding fluence value was 90 J/cm2. The experimental setup

chosen to produce nickel NPs in liquid media is shown in Figure 1.

The sample was moved using a XY motorized micrometric stage. The

movement was programmed so that the laser impinged always in different points of the

sample. In this way, ablation was performed always from a fresh surface of the sample.

This process lasted 9 min, after which the large number of NPs generated in the

surfactant free suspension produced a greyish colour in n<heptane or whitish in water.

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Figure 1. Schematic diagram of experimental setup for the fabrication of nickel colloidal suspension.

AFM images were recorded in air, at room temperature, and at a scan rate of

1 Hz, using standard semicontact mode of an NT<MDT Solver Pro microscope.

Rectangular shaped semi contact<mode cantilevers coated with Al on the reflex side,

having a spring constant of 37 < 58 N/m and holding a pyramidal silicon nitride tips of

tip curvature radius of 6 nm were used for topographical measurements. APPNANO

ACTA and ACLA probes at resonant frequencies of 273 kHz and 166 kHz were used

for samples in n<heptane and in water respectively. AFM provides a three<dimensional

surface profile. Lateral dimension are distorted by tip<sample convolution effects.

However, high z accuracy is obtained when height measurements with minimum

scanning vertical step equal to 0.012 nm are recorded. For these measurements, part of

the as<prepared colloidal suspension was sonicated and diluted to 1/1000 (v/v) in

n<heptane and 1/100 (v/v) in water. A drop of such diluted sample was placed on a

freshly cleaved muscovite mica sheet V<1 grade (SPI Supplies) and dried for 12 h at

room temperature. The analysis of mica sheet measurement indicated an average

roughness of 0.0612 nm.

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Microscopy images and electron diffraction patterns were performed with a

TEM<MSC (JEOL 2100, acceleration voltage 200 KV) at Brazilian Nanotechnology

National Laboratory (LNNano). The as<prepared Ni was diluted in n<heptane or mili<Q

water and sonicated during 15 min. Samples were obtained by drying the solvent<

dispersed NPs on a carbon coated copper grids followed by plasma cleaning to avoid

possible organic residuals. Images of the samples on a single<tilt sample holder were

recorded/captured using a TV Gatan ES500W and a CCD (TVips<16MP) camera.

OES was implemented by means of a Shimadzu spectrophotometer from 250 nm

to 1100 nm wavelength range. Optical absorption measurement of the colloidal

suspension in each liquid was performed 5 min after fabrication. This in situ

measurement prevents possible NPs coalescence and provides reliable statistics.

III. THEORETICAL BACKGROUND FOR EXTINCTION SPECTRA

CALCULATION

As it is well known, Ni is a ferromagnetic metal. In accordance to the analysis

made by Rosensweig,25 the frequency dependence of monodomain magnetic NPs

complex susceptibility follows a Lorentzian law with frequency and relaxation time:

�� = �� , where �� is the static field susceptibility, and � is the magnetic

relaxation time. For the case of single domain Ni NPs, the value of � is typically in the

range 10–2 < 10–6 s,26 which, for visible frequencies (1014 < 1015 Hz) yield �� very

close to zero. Under these conditions, the relative permeability is unity (�� = 1)

allowing the NP extinction to be correctly described by Mie theory24 for non magnetic

NPs.

In general, light extinction is the sum of absorption and scattering processes that

can be studied using the mentioned theory. For the general case of coated spherical

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particles with core radius � and outer radius �′, the extinction cross section can be

expressed as:

where �� and �� are the scattering coefficients which depend on the first, second and

third order spherical Bessel functions. The argument of these functions depends on the

core and shell size parameters and their refractive indices (see Supporting Information).

Sometimes, the so called extinction efficiency, defined as �� = ��/��′�, is used

for easiness of comparison.

The refractive index of materials is related to the dielectric function. For the case

of metals, the experimental bulk complex dielectric function (� !"#��) may be

modeled as an additive contribution of free and bound electrons. Free electron

contribution is usually written based on the Drude model, while bound electron

contribution may be written considering interband transitions:

� !"#�� = �$�� +� &!�'�� = �(�� + )�(′�� (2)

where free electrons contribution can be written as:

�$�� = 1 − �,�� + )-$��

(3)

and bound electron contribution as:

� &!�'�� = � !"#�� − .1 − �,�� + )-$��/

(4)

�� = 2�1� �2 34�25 + 1�� + ��6

�7�8

(1)

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In these expressions, �, is the bulk plasma frequency and -$�� is the damping constant

of the electron oscillatory movement. The latter represents the damping of electron

oscillation due to different processes: electron<electron collisions, electron<ion

collisions and electron<phonon collisions. These processes are characterized by the

mean time between two successive collisions and are taken into account altogether in

the -$�� value, since they cannot be specifically separated within the parameterization

of the bulk dielectric function.

When the size of metal NPs is smaller than the electron mean free path, the

dielectric function must be size corrected. The new expression may be written adding

size corrective terms for free and bound electrons to the experimental bulk dielectric

function:

�9:��, �� = � !"#�� + <�$��, �� + <� &!�'��, �� (5)

where

<�$��, �� = �,� = 1�� + )-$�� − 1�� + )-$�� + )�� >?@ A

(6)

and

<� &!�'��, �� = −2BC DDEF .� !"#�� − 1 + �,�� + )-$��/

(7)

In equation (6) GH is the Fermi velocity,� is a constant that depends on the

material and on the electron scattering processes at the particle boundary. It has a mean

value of 0.8.27

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In equation (7), the factor 2BC DDEF is related to the larger energy level separation

(and consequently a minor density of states) when the particle goes to smaller sizes

while the number of atoms decreases.28

Both size corrections <�$��, �� and <� &!�'��, �� in equation (5) depend on

Fermi velocity and Drude model parameters �, and -$��. The procedure to determine

the two latter parameters was described in detail in the work by Mendoza Herrera et

al.29 Briefly, two linear parametric equations involving the real and imaginary parts of

�$�� can be derived from equation (3). When these equations are plotted with the

experimental frequency as a parameter, linear regressions may be obtained only for

large wavelengths where Drude model is dominant. The slopes of these regressions

yield �, and -$��. Using the experimental values for the complex refractive index of

Ni reported by Ordal et al. in the wavelength range from 670 nm to 80000 nm,30

�, = (6.41 ± 0.12) × 1015 �K and -$�� = (3.16 ± 0.35) × 1013

�K were determined.

Figure 2 shows the real and imaginary parts of Ni size<dependent dielectric

function versus wavelength for different radii considering the bulk refractive index data

given by Rakić et al.,31 which coincides with Ordal et al. in the NIR<mid IR but extends

the data to wavelengths smaller than 670 nm. Theoretical calculations were performed

based on equations (5) to (7), using �, and -$�� mentioned above, � = 0.8,

GH = 2.8 × 1014 LMK ,32 and �& = 0.35 nm.

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Figure 2. Calculated values of real and imaginary parts of Ni size<dependent dielectric function vs wavelength for different radii using equations (5) to (7).

The experimental bulk dielectric function is represented by the curve in dashed

line. The curve for 3 nm radius (full line) is almost superimposed to that for bulk,

indicating that for larger radii (� > 3 nm) size corrections are negligible. However, for

sizes smaller than 3 nm, curves depart from the bulk dielectric function in the

considered wavelength range between 200 nm and 1000 nm.

Figure 3 shows the extinction efficiency �� of bare Ni NPs in water for radii

between 2 nm and 60 nm, plotted as a function of wavelength using equation (1). A

plasmon resonance for 7 nm radius can be observed as a small shoulder on the

extinction curve. This resonance is progressively redshifted up to 550 nm and increases

in intensity as the radius increases from 7 nm to 60 nm.

0

8

16

24

32

Wavelength [nm]

200 300 400 500 600 700 800 900 1000

ε (ω

, R)

<20

<15

<10

<5

0

0.6 nm 0.8 nm 1 nm 3 nm bulk

ε'

ε''

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Figure 3. Extinction spectra of Ni bare core NPs immersed in water. Plasmon resonance is redshifted and increases in intensity as the NPs radius increases.

A similar behavior in plasmon resonance redshift is observed when extinction is

calculated for bare Ni NPs in n<heptane.

If the particles are core<shell type, the extinction efficiency depends on the inner

radius � and the outer radius �′ (see Supporting Information). Figure 4 shows the

extinction spectra of core<shell air<Ni NPs in water for a core radius � = 10 nm and

different percentages of shell thicknesses. For this particle size, the shape of the spectra

is still due mainly to absorption, since scattering contribution is negligible.

Wavelength [nm]

300 400 500 600 700 800 900 1000

Ext

inct

ion

0

1

2

3

42 nm

7 nm11 nm15 nm20 nm30 nm60 nm

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Figure 4. Extinction spectra for core<shell air<Ni NPs in water for core radius � = 10 nm and different shell thicknesses.

The plasmon resonance that appears around 400 nm for 100 % shell thickness

redshifts as the shell thickness decreases, falling in intensity and broadening.

When the Ni target is ablated in water, oxidation<reduction processes may

occur.18, 33 In this case, it is necessary to consider the possibility of generating core<shell

NPs of the type Ni<NiO and NiO<Ni. Under these conditions, it is important to calculate

their extinction spectra, together with its dependence on shell thickness. Figure 5a

shows the extinction spectra of core<shell Ni<NiO NPs in water for a core radius

� = 10 nm and different percentages of shell thicknesses. As the NiO shell increases,

the plasmon resonance at 400 nm is slightly redshifted while it decreases in intensity.

Figure 5b shows the extinction spectra of core<shell NiO<Ni for the same core radius

and several shell thickness. In this case, the plasmon resonance is strongly redshifted as

Wavelength [nm]

300 400 500 600 700 800 900 1000

Ext

inct

ion

0.0

0.5

1.0

1.5

2.0

2.5

3.0

shell 10 % Rshell 20 % R shell 30 % R shell 50 % R shell 100 % R

R = 10 nm

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the shell thickness decreases, producing a broadening of the resonance. This behavior is

similar to that of dielectric core<noble metal shell NP.34 In both cases, dielectric

function of NiO was taken from Mahmoud et al.35

Wavelength [nm]

300 400 500 600 700 800 900 1000

Ext

inct

ion

0.0

0.2

0.4

0.6

0.8

1.0

1.2

shell 10 % Rshell 20 % Rshell 30 % Rshell 50 % Rshell 100 % R

R = 10 nm

(a)

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Figure 5: Optical extinction spectra calculated for (a) Ni<NiO and (b) NiO<Ni core<shell NPs in water, for core radius � = 10 nm and different shell thicknesses.

If the surrounding medium is n<heptane instead of water, a similar behavior in

plasmon tuning with shell thickness is observed.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

a. Ni Colloidal Suspension in n-Heptane

Experimental extinction spectra of Ni colloidal suspension obtained by fs pulse

laser ablation in n<heptane were performed immediately after preparation. Figure 6

shows a typical extinction spectrum of Ni NPs suspension in n<heptane normalized at

λ = 340 nm. Full line represents the experimental curve while dashed<dotted line

represents the fit according to the size distribution shown in the inset. The particle

Wavelength [nm]

300 400 500 600 700 800 900 1000

Ext

inct

ion

0.0

0.5

1.0

1.5

2.0

shell 10 % Rshell 20 % Rshell 30 % Rshell 50 % Rshell 100 % R

R = 10 nm

(b)

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sizing is represented in it by log<normal size distributions of bare core Ni NPs with

modal radii at 2.5 nm and 10 nm together with air<Ni core<shell NPs at external modal

radii of 6.6 nm (� = 6 nm; shell 10 % �), 12 nm (� = 10 nm; shell 20 % �) and 15.6 nm

(� = 15 nm; shell 4 % �).

Figure 6. Comparison between experimental (full line) and theoretical (dashed<dot line) extinction spectra of Ni colloidal suspension in n<heptane. Inset shows bare core and air<Ni relative abundance contributions that fit experimental spectrum. Long dashed line represents calculated spectrum with the same distribution without taking into account air<Ni NPs.

Based on the relative abundance curve shown in the inset of Figure 6 and taking

into account the Lambert Beer Law, we can calculate the number density (P) of each

species present in the colloidal suspension through a factor that relates the experimental

normalizing point at a given wavelength with the theoretical normalizing point at the

Wavelength [nm]

300 400 500 600 700 800 900 1000

Nor

mal

ized

ext

inct

ion

0.0

0.5

1.0

1.5

2.0

Experimental Theoretical fitTheoretical fit without air<Ni Nps

Radius [nm]0 5 10 15 20

Rel

ativ

e ab

unda

nce

0.0

0.1

0.2

0.3

0.4

0.5

Total distributionNi Nps contributionair<Ni Nps contribution

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same wavelength. The equation relating the number density P and the relative

abundance < (in cm–2 units) of the i<th species is P = QRSTU , where V is the experimental

spectrum normalization factor and W, the theoretical spectrum normalization factor. This

normalization can be carried out for any given wavelength. In our case V = 0.182 and

W = 114.95 for λ = 340 nm. In this way, it is possible to determine the number density of

bare core Ni and air<Ni NPs of different radii present in the colloidal suspension as

shown in Table 1. From OES measurements, the total number of NPs probed by the

spectrophotometer beam is approximately 2 × 1010. This value shows the high statistics

of OES technique.

Table 1. Number density and relative percentage of different species of NPs present in the colloidal suspension in n<heptane

Species Number density [cm–3

] Relative

percentage

Ni bare core 1.34 × 1011 82 %

Air<Ni core<shell 2.95 × 1010 18 %

A key contribution for the accurate fit of the experimental extinction spectrum

from 350 nm onward can be found in the extinction behavior of hollow<type NPs. As

shown for the aqueous suspension in Figure 4, the extinction spectrum corresponding to

air<Ni NPs for 10 nm inner radius and Ni thickness smaller than 10 % of core radius

shows a very broad resonance centered at about 700 nm. This feature contributes to fit

the extinction spectra for λ > 350 nm. Calculated extinction without considering the

contribution of the air<Ni NPs is represented by the dashed line in Figure 6. It is readily

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seen that the curve fits the experimental spectrum only up to 350 nm, departing

noticeably from it for larger wavelengths.

Air<metal core<shell structures have been reported by other authors by ablating

specific materials using different kind of pulsed lasers. For example, Desarkar et al.

obtained Zn/ZnO hollow NPs using a 6 ns Nd:YAG pulsed laser ablation of zinc in

liquids.36 Yan et al. fabricated hollow Pt and Al2O3 NPs using a 50 ns pulsed excimer

laser for ablating solid Pt and Al targets in water.37–39 These authors propose that the

hollow particles are formed at the boundary of laser<produced bubbles, which provide a

preferred nucleation site and diffusion sink for self assembly of the laser<fabricated

clusters. Niu et al. obtained hollow NPs of diverse metal oxides and sulfides by laser

ablating metal targets in specific liquids, based on the Kirkendall effect after heating

with an infrared laser.40 More recently, we have found this type of hollow structures

using 120 fs pulsed laser ablation of a silver target in water,41 apart from other kind of

species present in the colloidal suspension. Our results on silver ablation and on Ni

ablation with fs regime, suggest that a similar formation mechanism about laser induced

bubbles may produce the observed hollow NPs. Although the relative abundance of Ni

hollow NPs in the colloidal suspension is small (about 18 %, as indicated by the dash

dotted line in inset of Figure 6), their influence on the shape on the extinction spectrum

in the Vis<NIR region is noticeable as can be seen in Figure 6 (dashed line curve).

AFM images of single NPs were obtained to characterize shape and size of the

Ni colloidal suspension generated in n<heptane. Figure 7a shows 50 �m × 50 �m

scanned area image where small isolated NPs are observed. Height profiles scanned in

lines 1 to 5 show NPs with typical external diameters of 15 nm, 27 nm, 5 nm, 22 nm

and 23 nm respectively, as indicated in Figure 7b. For these small size values, it is

important to consider the roughness of the mica substrate on which the drop was

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deposited. Line 3 in panels (a) and (b) shows the height profile of the mica background

across the imaged area. It can be seen that the mean roughness is about one order of

magnitude smaller than the smallest recorded particle, so there is a very good signal<to<

noise ratio in the nanoparticle’s profile. Panel (c) shows the radius histogram of the NPs

in the image of panel (a). The histogram can be fitted by two log<normal size

distributions centered at modal radius �′M� = 2 nm and �′M� = 7 nm.

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X [�m]0 10 20 30 40 50

h [n

m]

0

5

10

15

20

25

30line 1 line 2line 3 line 4 line 5

(b)

Radius [nm]

0 4 8 12 16 20

Cou

nts

[a.u

.]

0

5

10

15

20

25

30

Radii distributionTotal distributionLog Normal R'm1 = 2 nm; σ

1 = 0.4

Log Normal R'm2 = 7 nm; σ2 = 0.4

(c)

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Figure 7. AFM size analysis of the obtained Ni colloidal suspension in n<heptane: (a) image of isolated single Ni NPs; (b) height profiles of selected lines 1 to 5 from panel (a) which correspond to the spherical particles diameter; (c) radius histogram together with two log<normal size distributions at �′M� = 2 nm and �′M� = 7 nm. Full line represents the sum of both distributions.

Although the amount of particles observed in the AFM image is much smaller than that

probed by OES, it can be seen that the heights in panel (b) and the histogram in panel

(c) of Figure 7 are in good agreement with the external radii distribution determined

from the fit of the extinction spectrum of the sample shown in the inset of Figure 6.

To study the morphology of Ni NPs, TEM analysis was carried out. Figure 8

shows TEM images of Ni colloidal suspension fabricated by ablation in n<heptane.

Panel (a) shows a panoramic view of isolated bare core and hollow Ni NPs. Panel (b) is

an enlargement of a bare core Ni NP from panel (a). Panel (c) is another panoramic

view together with an enlargement of an air<Ni NP. Dotted line outlines the limit of the

core and shell structure. Panel (d) shows the radius histogram corresponding to a

statistics performed over several TEM images. The results can be fitted by two log<

normal size distributions which describe the most prominent features of the histogram.

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Figure 8. TEM image of NPs present in the Ni colloidal suspension in n<heptane. (a) Panoramic view; (b) single isolated bare core Ni NP; (c) detail of a hollow Ni NP and (d) radius histogram and log<normal size distributions which account for the most prominent features of the histogram.

It is interesting to notice that the sizes arising from TEM analysis (Figure 8) as

well as those obtained from AFM (Figure 7) are in agreement with results derived from

OES shown in Figure 6. Spherical bare core and core<shell structures can be seen. This

fact supports the use of Mie approach for calculating extinction spectra as well as the

inclusion of this type of hollow particles for fitting the spectrum.

b. Ni Colloidal Suspension in Water

Ni colloidal suspensions in water were studied with the same methods as before.

Figure 9 shows a typical extinction spectrum in the range UV<visible<near IR

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normalized at λ = 340 nm. Full line represents the experimental extinction curve, while

dashed<dotted line is the theoretical fit. In this case, since the solvent is water, some

degree of oxidation is surely to occur. Structures and configurations like bare core NiO

besides core<shell structures of Ni<NiO, NiO<Ni as well as air<Ni were considered to be

present in the colloidal suspension. The inset shows in full line the total size distribution

that yield the optimum fit of the experimental spectrum. Specific size distribution for

Ni<NiO NPs (dotted line), air<Ni NPs (dashed<dotted line) and NiO<Ni NPs (dashed

line) are also shown. The latter size distribution includes bare core NiO NPs species. It

can be noticed that the multimodal size distribution shape is similar to that determined

for n<heptane but shifted to larger values of external radii. This fact may be due to the

presence of oxide shells around the NPs. Each one of the previously mentioned species

influence the extinction spectrum in specific and distinct regions in such a way that the

combination of structures, configurations, sizes and relative abundances derived from

optimum fit constitute a unique set of fitting parameters. For instance, theoretical

calculation without considering air<Ni NPs, will not fit the experimental extinction

spectrum for wavelengths larger than 400 nm (long dashed line in Figure 9). Similarly

to the effect shown for n<heptane, the exclusion of this type of particles precludes the

correct fit of the spectrum for this wavelength range. Notice that, although the

abundance of air<Ni NPs is rather small (see inset), it has a very large effect in the

fitting procedure. A similar lack of fitting is observed when the contribution of NiO<Ni

NPs is not considered (not shown in Figure 9). For a given species type any small

variation in size or relative abundance produce a noticeable change in the shape of the

spectrum.

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Figure 9. Comparison between experimental (full line) and theoretical (dashed<dots line) extinction spectra of Ni colloidal suspension in water. Inset shows size distribution of Ni<NiO, air<Ni and NiO<Ni NPs that fits experimental spectrum. Long dashed line represents calculated spectrum with the same distribution without considering air<Ni NPs contribution.

As in the case of n<heptane, OES technique allows to calculate the number

density of the different structures and configurations present in the colloidal suspension

in water. Considering the relative abundance curve shown in the inset of Figure 9,

V = 0.553 and W = 488.323 for λ = 340 nm it is possible to calculate the number density

of Ni<NiO, Air<Ni and NiO<Ni core<shell species present in all sizes in the colloidal

suspension fabricated in water. Results are shown in Table 2

Wavelength [nm]

300 400 500 600 700 800 900 1000

Nor

mal

ized

ext

inct

ion

0.0

0.5

1.0

1.5

2.0

Experimental Theoretical fitTheoretical fit without air<Ni Nps

Radius [nm]0 10 20 30 40

Rel

ativ

e ab

unda

nce

0.00

0.04

0.08

0.12

0.16

0.20

Total distributionNi<NiO Nps contributionair<Ni Nps contributionNiO<Ni Nps contribution

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Table 2. Number density and relative percentage of different species of core<shell NPs present in the colloidal suspension in water

Species Number density [cm–3

] Relative

percentage

Ni<NiO 8.38 × 1010 75 %

Air<Ni 1.02 × 1010 9 %

NiO<Ni 1.77 × 1010 16 %

As in the case of Ni NPs in n<heptane, hollow NPs are present in the water

colloidal suspension. Its formation could be related to the laser induced bubbles. The

optical breakdown induced by laser ablation in water, produces H2 gas as by<product

from the following reaction

Ni + H2O → NiO + H2 (12)

Besides, O2 and H2 may also be generated in water at the focus of the laser beam. The

gas content increases the stability of the bubbles,39 providing a larger probability for

hollow particle formation. Since during fs laser ablation high temperatures and

pressures are easily reached in the plasma plume and in the plasma<liquid interface42

during a time interval compatible with the fast oxidation kinetics at the nanoscale (about

100 ns), it is possible to argue that, under our experimental conditions, the previous

reaction may generate oxidized species in the form of NiO and Ni<NiO NPs in the

colloidal suspension.

There is another process that must be considered. Medford et al. reported that

when hollow oxidized Ni NPs in the size range 10 nm to 90 nm are exposed to a low

content H2 atmosphere at 350 ºC, they undergo a reduction process (which depends on

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the initial size of the NPs) that gives rise to a metal shell around the particles.43 As

stated above in equation (12), during fs laser ablation of Ni, H2 is produced as a by<

product that, together with the high metal surface temperature obtained in the ablation

process, allows the reduction of the formed NiO NPs, yielding a core<shell species of

the type NiO<Ni in the colloidal suspension.

Figure 10a shows an AFM image of 30 �m × 25 �m scanned area of isolated

NPs in water colloidal suspension, dried on fresh mica surface. Height profiles scanned

in lines 1 to 6 show NPs with typical external diameters of 15.9 nm, 86.3 nm, 6.4 nm,

24.5 nm, 43.8 nm and 10 nm respectively, as indicated in Figure 10b. Line 3 shows the

height profile of the mica background across the imaged area, which, as stated for

Figure 7b, is much smaller than the measured NPs.

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Figure 10. AFM size analysis of the obtained NPs after ablation of Ni target in water: (a) image of isolated NPs; (b) height profiles of selected lines 1 to 6 from panel (a).

Figure 11 shows TEM images of Ni colloidal suspension obtained by ablation in

water. Panel (a) shows an isolated air<Ni core<shell NP. Panel (b) shows a bare core and

core<shell NPs along with their respective sizes, placed in a different plane with respect

to the other agglomerates of NPs. This is the reason why it is possible to observe other

smaller NPs beyond the particle as the TEM electrons traverse across the sample. Panel

(c) is a panoramic view where spherical particles showing different structures can be

observed. Panel (d) shows the radius histogram corresponding to a statistics performed

over several TEM images. Some isolated NPs with radius larger than 40 nm, which are

shown in these images for illustrative purposes, are not plotted on the histogram

because they represent less than 1 % of the observed number of particles.

X [�m]

0 5 10 15 20 25 30

h [n

m]

0

20

40

60

80

100

line 1line 2line 3line 4line 5line 6

(b)

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The results can be fitted by two log<normal size distributions which describe the

most prominent features of the histogram. As in the case of n<heptane, there is a good

agreement between TEM, AFM and OES results.

Figure 11. TEM image of Nps present in Ni colloidal suspension in water. (a) isolated single hollow Ni NP; (b) spherical bare core and core<shell NP. (c) panoramic view showing bare core and core<shell NPs and (d) radius histogram where two log<normal size distributions account for the most prominent features of the histogram.

TEM images of the NPs obtained by laser ablation are shown in Figure 12. Panel

(a) contains enlargements of two regions where NiO Bragg planes can be observed

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(Table 3). The NPs shown in Figure 12 are very similar to those reported by Medford et

al. at the beginning of the reduction process by exposure to H2.43

Electron diffraction patterns were measured on selected NPs for phase

identification. A representative pattern is shown in Figure 12b, which is indexed with

the reflection lines of NiO (Cubic, mFm3 , JCPDS #75<0197) and Ni (Cubic, mFm3 ,

JCPDS #04<0850). Electron diffraction rings are labeled according to Table 3, where the

Miller indices (h, k, l) and the interplanar distances (d) are indicated. No reflection

characteristics of other structures appear.

Figure 12. (a) TEM image of Ni NPs obtained by laser ablation in water; enlargements show typical regions where NiO Bragg planes can be observed, (b) Electron diffraction pattern indexed with the reflection lines of NiO (lines 1, 2 and 5) and Ni (lines 3 and 4) according to Table 3.

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Table 3. Interplanar distances (d) and Miller indices (h, k, l) for Ni (JCPDS #04<0850) and NiO (JCPDS #75<0197) used to index the electron diffraction pattern

Ring dNi (nm) dNiO (nm) (h, k, l)

1 0.2369 (1, 1, 1)

2 0.2052 (2, 0, 0)

3 0.2034 (1, 1, 1)

4 0.1762 (2, 0, 0)

5 0.1451 (2, 2, 0)

V. CONCLUSIONS

Colloidal Ni NPs were generated in n<heptane and water after fs pulsed laser

ablation on solid target. The suspensions were analyzed using spectroscopic,

microscopy and electron diffraction techniques to characterize their structure,

configuration, composition and sizing. NPs resulted spherical in shape, with bare core

or core<shell structure.

Fit of the experimental extinction spectra in each solvent using an iterative

method based on Mie theory with appropriate size correction of the bulk complex

dielectric function yielded the size distributions of bare core and core<shell NPs in the

Ni colloidal suspension. For n<heptane, the total log<normal size distribution had a

modal radius of 2.5 nm with a secondary maximum at 7 nm and 12 nm. This

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distribution consists of two different structures: bare core and hollow Ni NPs. For water,

the total log<normal size distribution had a modal radius of 4 nm with a secondary broad

distribution centered at 20 nm. In this case, only core<shell structures composed by

hollow Ni, Ni<NiO and NiO<Ni NPs contribute to the total distribution. The formation

of NiO<Ni and hollow Ni species is reported for the first time in this paper.

In spite the number of particles observed in the AFM image is much smaller

than that probed by OES (which is of about 1010), the AFM heights (diameters)

histogram is in good agreement with the external radii distribution determined from the

fit of the extinction spectrum. TEM analysis yielded spherical bare core, metal<metal

oxide core<shell structures and hollow air<Ni NPs. This fact confirms the results

obtained using Mie theory with the inclusion of core<shell and hollow particles for

fitting the spectra. Since OES provides a non local analysis of the samples, as compared

with TEM and AFM, we consider that OES yields statistically reliable results about size

distribution and structures.

Electron diffraction patterns were measured on selected NPs for phase

identification. The obtained representative pattern was indexed with the reflection lines

of NiO and Ni, thus also supporting the species distribution obtained after the optical

absorbance spectrum fitting. The formation of Ni<oxide core<shell species may be

explained by the generation of oxygen atoms during laser ablation in water and

subsequent metal oxidation processes. Metal shell formation in NiO<Ni NP may arise

due to exposure of NiO NP to a low<content H2 environment due to laser photolysis. On

the other hand, hollow Ni NPs may be produced during bubble formation in laser

ablation.

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VI. ASSOCIATED CONTENT

Supporting Information

Equations for theoretical calculations of extinction cross section, as well as full

description of references 8 and 17, have been provided. This material is available free of

charge via the Internet at http://pubs.acs.org.

VII. AUTHOR INFORMATION

Corresponding Author

*Phone: +54 221 4840280/4715249 int. 213. Fax: +54 221 4712771. E<mail:

[email protected].

Notes

The authors declare no competing financial interest.

VIII. ACKNOWLEDGMENTS

This work was granted by PIP 0394 and PIP 0720 of CONICET, PME2006<

00018 of ANPCyT, grant 11/I151 of Facultad de Ingeniería Universidad Nacional de La

Plata and grant 11/X680 of Facultad de Ciencias Exactas Universidad Nacional de La

Plata, Argentina. We thank C2NANO < Brazilian Nanotechnology National Laboratory

(LNNano) at Centro Nacional de Pesquisa em Energia e Materiais (CNPEM) / MCT

(#14825 and 14827) and Research Proposal TEM<16976 for the use of TEM. AFM was

carried out at LFAyM of Instituto de Física La Plata (IFLP < CONICET). We

acknowledge Dr. Francisco Sánchez from Instituto de Física La Plata (IFLP <

CONICET) for fruitful discussions on magnetic nanoparticles.

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D. C. Schinca is Member of Comisión de Investigaciones Científicas de la

Provincia de Buenos Aires (CIC). L. B. Scaffardi, M. B. Fernández van Raap, P.

Mendoza Zélis and J. M. J. Santillán are researchers of CONICET. D. Muraca is

Member of Instituto de Física “Gleb Wataghin” (IFGW), Campinas, Brazil. D. Muñetón

Arboleda and L. J. Mendoza Herrera are PhD fellows of CONICET, Argentina.

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Graphic Abstract

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Ablation in Liquids: Structure and Sizing Synthesis of Ni ...

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